1. Field of the Invention
This invention relates to an oxygen concentration detection device which is equipped with a function to detect whether an oxygen sensor installed in an exhaust pipe is malfunctioning or not.
2. Description of Related Art
In conventional A/F ratio feedback control systems which feed back the A/F ratio based on the output signal from an oxygen sensor installed in the exhaust pipe of an automobile, it is determined that the oxygen sensor is malfunctioning when the oxygen sensor output drops below a specified oxygen sensor output level or becomes negative to generate a warning to a driver, as disclosed in Japanese Patent Laid-open No. SHO58-139548, SHO52-46890 and HEI4-224250.
Oxygen sensors which are generally used for A/F ratio feedback control are equipped with an exhaust electrode which contacts exhaust gases flowing through the exhaust pipe and an air electrode which contacts the air. Following this, a voltage is generated between the both electrodes according to the oxygen concentration differential between the air and the exhaust gas electrodes to detect the oxygen concentration in exhaust gas emissions based on the voltages. Normally, the oxygen concentration at the air electrode side is identical to that of the air in the atmosphere. However, when combustible substances come into contact with the air electrode, the oxygen concentration decreases rapidly due to combustion (oxidation reaction) of the combustible substances through exhaust gas heat, which causes an oxygen deficiency to occur at the air electrode side. At the same time, the oxygen concentration on the air electrode side and the exhaust gas electrode side is reversed, which causes the electrical output of the oxygen sensor to become negative, even when the oxygen sensor is not malfunctioning. This may lead to the erroneous determination that the sensor is malfunctioning.
As combustible substances that way give rise to errors such as above, factors such as dust in the air and residual fuel in exhaust gas emissions are considered as potential causes. Residual fuel contained in exhaust gas can adhere to the surface of the exhaust electrode of the oxygen sensor exposed inside the exhaust pipe penetrate a sensor seal portion and finally leak outside to enter into the air electrode. This phenomenon has been proven through experiments by the inventor of the present invention. However, from the viewpoint of the structure of oxygen sensors, it is difficult to protect the air electrode from coming into contact with combustible substances. Under these circumstances, the most important technical matter remaining to be solved at the present stage of development is how to prevent the oxygen sensor from being erroneously detected as malfunctioning due to combustible substances in exhaust gas emissions.